The present invention relates generally to optical communications, and, more particularly, to precisely arranging optical elements in a two-dimensional array. Certain embodiments of the invention provide for precisely configured arrays of optical elements, such as optical fibers and lenses.
Optical communication systems have been in existence for some time and continue to increase in use due to the large amount of bandwidth available for transporting signals. Optical communication systems provide high bandwidth and superior speed and are suitable for efficiently communicating large amounts of voice and data over long distances. Optical communication systems are typically employed for both long and short distance communications applications, but are generally most efficient when used for long distance communications. In a typical optical communication system, spans of optical fibers are connected by switching systems located along the fiber spans. These switching systems are used to both route the optical signals to their destination, and to add and remove optical signals from the optical fibers. Some optical switches require that the optical signal first be converted to an electrical signal, then switched and converted back to an optical signal. Other switching systems switch the optical signal while the signal remains in the optical domain.
One manner of switching optical signals uses a number of movable mirrors to route the optical signal through the optical switch from an input fiber to an appropriate output fiber. Such an optical switch typically receives signals from a large number of optical fibers and requires that the light from each fiber accurately impinge on an appropriate mirror. Supplying optical signals to an optical switch is sometimes referred to as xe2x80x9claunchingxe2x80x9d light into an optical switch. The light is referred to as being launched into free space because the light travels toward the mirror without the aid of a waveguide. Unfortunately, when many optical fibers are associated with such an optical switch, accurately aligning each of the input fibers with a corresponding mirror so that the optical signals are accurately launched into the optical switch becomes difficult.
One possible manner of aligning optical fibers involves etching recesses, or grooves, into a substrate. The optical fibers are located and retained in the grooves. Multiple etched substrates may be stacked over one another and joined together, thus forming a two dimensional array of optical fibers. Unfortunately, due to the thickness variation between substrates and the significant thermal coefficient of expansion (TCE) (sometimes referred to as coefficient of thermal expansion (CTE)) of the bonding material, the mechanical tolerances of such a system are difficult to control with precision over time and temperature variations. This alignment difficulty limits the number of optical signals that can be supplied to such an optical switch.
Therefore, there is a need in the industry for accurately aligning fibers in arrays so that the output of each input fiber accurately impinges on an input mirror of an optical switch and so that each light from each output mirror of an optical switch accurately impinges on a corresponding output fiber.
The present invention provides for aligning elements, such as optical fibers and lenses, by inserting them into holes formed in a wafer. The wafer can be a thin slice of solid material; the holes can be formed using any known technique in the pattern desired for arranging the optical elements. The wafers can be silicon wafers such as those commonly used in semiconductor manufacturing; the holes can be formed using known photolithographic silicon etching techniques.
Two such wafers can be made and aligned with each other so that one array of optical elements, e.g., optical fibers, can be aligned with another array of optical elements, e.g., lenses. Thus, an optical switching system can comprise two pairs of wafers coupled by an optical matrix switch. The first pair of wafers aligns input fibers to collimating lenses that launch light into the matrix switch. The second pair of wafers aligns focusing lenses to optical fibers so that light from the matrix switch can be directed into the fibers for transmission elsewhere. The matrix switch determines the coupling between input fibers and output fibers.
The invention provides for a method for aligning optical elements. The method involves forming a plurality of through holes in a wafer, and inserting optical elements into the holes. Typically, the holes are configured in a two-dimensional array that defines the alignment of the optical elements. Preferably, the holes are etched into the wafer. For example, reactive ion etching can be used to form holes on one surface of the wafer; the holes can be converted to through holes by wet etching into the opposing surface of the wafer. Where fibers are inserted into the through holes, they can be polished to be coplanar with a first surface of the wafer, preferably a dry-etched surface. The holes can be tapered to guide fibers into proper alignment upon insertion.
The present invention provides a precise, reliable, and economical approach for aligning large numbers of fibers in a two-dimensional array and for aligning such an array of fibers with an array of lenses. The wafers can be well-characterized widely-available silicon wafers and the holes can be made precisely using mature semiconductor manufacturing methods. Fiber alignment is facilitated by the tapered shape of the holes. Concerns regarding mechanical tolerances of layered substrates in the prior art are substantially obviated by the present invention. From a system perspective, the invention provides for much more reliable matrix switching for large numbers of optical paths; light from input fibers accurately impinge on switch input mirrors, while light from output mirrors is properly aligned with target output fibers. Other advantages in addition to or in lieu of the foregoing are provided by certain embodiments of the invention, as is apparent from the description below with reference to the following drawings.